The present invention relates to a method and means for increasing the frequency of update of direction information contained in two, essentially identical, alternating current signals, essentially sinusoidal in waveform, and which are in quadrature. Although the invention is described as receiving sinusoidal information produced by a rotary magnetic encoder, such is for illustrative purposes only and it will be understood that the invention is applicable to any case where it is desired to increase the frequency of update of direction information contained in two signals of similar characteristics.
A rotary magnetic encoder is a well-known device which typically includes a cylindrical rotating magnetic member, upon the outer circumference of which is a continuous series of N-S pairs of magnetic poles. A stationary member holds one or more detectors so disposed that, as the rotating member rotates, the flux at successive magnetic poles links those poles with the detector and the detector produces a characteristic alternating current sine wave output. Each full cycle of the sine wave represents the movement of one N-S pair of poles past the detector. Typically, it is found in decoding the sinusoidal information, that it is desirable to generate a digital pulse at each point at which the sine wave has a value of zero (or a "zero crossing"). Since a full cycle has two zero crossing, there will be generated two pulses for each full sine wave. It is common to employ two detectors on the stationary member, displaced 90 electrical degrees from one another with respect to the magnetic poles, in order to provide direction information and in order to double the number of total pulses per cycle for increased resolution. Direction information is obtained from the two sine waves through conventional means, by detecting which sine wave is leading the other. Thus, direction information is updated four times per revolution.
Conventional techniques are known for increasing the number of pulses per cycle to eight or more, but, in the process of so doing, direction information is lost. Accordingly, it is an object of this invention to provide a method and means to increase the frequency of update of direction information to eight times per full sine wave. The present invention accomplishes this in a novel manner by generating eight rectangular waveforms from the two primary sine waves. The derivatives of the eight rectangular waveforms are then taken and combined with the rectangular waveforms from which they are derived into two sets of eight additive pairs each, with a high level logic signal from one set indicating one direction and a high level logic signal from the other set indicating the opposite direction. The direction information is updated eight times during each full cycle of each primary sine wave.
It will be understood that, in accordance with conventional nomenclature, "sine wave" as used herein means an electrical signal essentially sinusoidal in waveform, and "rectangular waveform" as used herein means an electrical signal essentially rectangular in waveform.